1. Field of the Invention
Aspects of the present invention relate to electrolytes for lithium secondary batteries and lithium secondary batteries including the electrolyte, and more particularly, to an electrolyte for a lithium secondary battery and a lithium secondary battery including the electrolyte that may improve lifetime characteristics and high-temperature durability characteristics of the lithium secondary battery.
2. Description of the Related Art
Typical rechargeable secondary batteries include lead storage batteries, nickel-hydrogen batteries, and lithium secondary batteries. The lithium secondary batteries, due to the nature of lithium, which is the lightest metal in the periodic table, have a high energy density per unit weight and an electromotive force of 4 V, which is three times as high as that of nickel-based batteries, and thus may be manufactured smaller. The lithium secondary batteries have no memory effect with reduced energy capacity even when charged in an incompletely discharged state. Due to these characteristics, lithium secondary batteries have been extensively used in devices such as mobile products, laptop computers, and electric tools. Lithium secondary batteries are expected to be available as major power sources for electric vehicles and power storage.
A lithium secondary battery includes a cathode, an anode, a separator, and an electrolyte. Lithium ions are separated from the cathode and migrate to the anode during charging, while the lithium ions are separated from the anode and migrate back to the cathode during discharging.
The electrolyte, which is one of the main elements of the lithium secondary battery, may serve as a migration medium (conductor) of lithium ions that are generated from electrochemical reactions in the cathode and the anode. The electrolyte may consist of a lithium salt and an organic solvent. The electrolyte of the lithium secondary battery is typically a non-aqueous system, which may be used to generate a high voltage due to a wide electrochemical stability window, even though it has low conductivity.
With the secondary battery market expanding into electric vehicle and power storage markets, lithium secondary batteries are adopting novel electrode active materials that ensure high energy density. However, the use of a low-potential anode active material and a high-potential cathode active material to increase the energy density has caused the electrolyte to be vulnerable to decomposition at the surface of the cathode and anode due to a narrowed potential window of the electrolyte relative to the active materials.
In a graphite anode, use of an appropriate electrolyte or an electrolyte additive is known to form a film on the surface of an anode active material during initial charging, which prevents the anode active material from directly contacting the electrolyte that would decompose the electrolyte. In a cathode, an electrolyte additive is known to work as an overcharging inhibitor by forming a thick film on the surface of the cathode when the voltage of the battery rises over a specific voltage, thus blocking passage of lithium ions and stopping operation of the battery.
With the recent use of cathode active materials possessing high voltage characteristics, there has been a growing demand for forming thin protective films on the cathodes. In this regard, research has shown that a thin film formed on the surface of a cathode by adding an overcharging inhibitor in a low concentration improves the lifetime of a battery (Electrochemical and Solid-State Letters, 7(12) A462-A465 (2004)). However, this thin film is non-polar, and thus hinders passage of lithium ions, adversely affecting battery characteristics.
Batteries for electric vehicles and power storage are more likely to be exposed to high-temperature environments, and the temperatures of the batteries are apt to rise during periods of instantaneous charging and discharging. Thus, these batteries need to operate normally at high temperatures. However, chemical activity of the electrolyte increases at high temperatures, causing unwanted additional chemical reactions, such as self-discharging, to occur.